1. Field of the Invention
The invention relates to a motor monitor switch for an electric motor which is reversible in its direction of rotation, with a direction of rotation reversal circuit which can be put in the power supply lines of the electric motor and with a monitoring circuit, which can be put in the power supply lines to be monitored between the direction of rotation reversal circuit and the electric motor, the direction of rotation reversal circuit having a plurality of reversing switch contacts, the monitoring circuit having at least one current transformer which can be put in the power supply line to be monitored, a monitoring circuit for the actual current value, an overload protection switch and a voltage supply circuit, where the monitoring circuit for the actual current value is connected with its input to a secondary coil of the current transformer and with its output to the overload protection switch and comprises an actual value analyzing step and a control step placed downstream of the actual value analyzing step, i.e. is disposed on the output side, and where the response level of the monitoring circuit for the actual current value can be varied.
2. The Prior Art
The known motor monitor switch on which the invention is based (German Published, Non-examined Patent Application DE-OS 27 04 820) can be used, because of its specialized design for use with electric motors with totally different rated currents, over a wide range of possible rated currents. In various other respects this motor monitor switch is also constructed in a particularly practical manner. For example, an appropriate delay circuit has been provided in consideration of a start-up current which mostly is much higher than the rated current, the overload protection switch is supplied with a set voltage by the voltage supply circuit, and the monitoring of the actual current value has been provided with a specially designed actual value analyzing step and a specially designed control step. In this connection, reference is made to German Published, Non-examined Patent Application DE-OS 27 04 820, the contents of its disclosure being made the contents of the disclosure of the present application by reference.
The known motor monitor switch can be used in connection with electric motors having a set direction of rotation as well as with electric motors with a reversible direction of rotation. In the latter case it is understood that a conventional direction of rotation reversal circuit is naturally a part of such a motor monitor switch, in this case such a motor monitor switch comprises a direction of rotation reversal circuit which can be put in the power supply lines of the electric motor as well as a monitor circuit which can be put in the power supply lines of the electric motor which are to be monitored. For example, in a three-phase a.c. motor, two of the three phases are monitored by the monitoring circuit, reversal of the direction of rotation of the electric motor being accomplished by a change of the two phases, which is of no consequence for the monitoring circuit. This poses no problem for electric motors operating symmetrically, i.e. electric motors in which the monitoring circuit of the motor monitor switch is intended to have one and the same response level for both directions of rotation. However, problems occur when it is intended to provide different response levels for the directions of rotation of the electric motor. The term response level should be understood in the broadest sense of the word, therefore it does not only mean the absolute value of a current which is perhaps being monitored but also, for example, the length of time a set value is exceeded, a combination of the two criteria or other relevant criteria.